Recent, rapid restructuring of North American bumble bee communities is associated with climate warming

A rapidly warming climate has become one of the primary forces driving changes in biodiversity worldwide. The impact of warming temperatures on insect communities is of particular interest given their importance for ecosystem function and service provision and the uncertainty around whether insect communities can keep pace with the rate of increasing temperatures. We use a long-term dataset on bumble bee species occurrence and data on summer maximum temperature trends across North America to characterize community-level responses to recent climate warming. Bumble bees are relatively well recorded historically and are sensitive to warming temperatures. We examined responses using the community temperature index (CTI) – a measure of the balance of cool- and warm-adapted species within local communities. Starting in 2010, bumble bee average CTI across North America has rapidly increased after a period of slight increase from 1989 to the late 2000s. This increase is strongly associated with recent increases in maximum summer temperatures. The increase in CTI is spatially extensive, occurring throughout North America, but the areas of greatest concern include mid to high latitudes as well as low and high elevations - areas relatively shielded from other intensive global changes (e.g., land-use). On average, bumble bee CTI has increased 0.99°C from 1989 to 2018, a change of similar magnitude to the increase in maximum summer temperatures. The rapid shift in bumble bee communities appears to be at pace with shifting summer temperatures, with an approximate, equivalent northward shift of ~104 km from 1989-2018 for both. This indicates an adaptive capacity among some bumble bee species. However, warming temperatures are also likely reducing the occurrence and local abundance of cool-adapted species that may serve important ecological roles within their range. Our results provide strong evidence of the pervasive impacts posed to insect communities by temperature increases in the past few decades.